1. Field of the Invention
This invention relates to apparatus for passively tracking a target emitting radio signals and more particularly to tracking by sequential lobing a target emitting a modulated radio signal by sequential lobing.
2. Description of the Prior Art
One method of obtaining meteorological data is by launching a radiosonde which typically includes a balloon, sensors and a radio transmitter. The radiosonde may be launched from the earth's surface. As the radiosonde ascends, the temperature, relative humidity and atmospheric pressure are transmitted by a radio transmitter. The transmitted signals may have a predetermined format and may be sent by a frequency modulated carrier such as at 1680 MHz.
The radiosonde may be tracked by an antenna on the ground which is sequentially lobed about the target to provide an error signal to position the antenna. The pressure information transmitted by the radiosonde gives an indication of altitude. The tracking angle or position of the target over time provides a means for determining the wind velocity and direction as a function of altitude.
Presently the radiosonde may be tracked with an antenna having four dipoles mounted about the focal point of a parabolic reflector. The microwave signals received by the dipole antennas may be combined to provide an amplitude signal, indicative of the off boresight location of the target emitting radio signals. One example of a mono-lobed scanner or antenna suitable for tracking a radiosonde is described in U.S. Pat. No. 3,983,562 which issued on Sept. 28, 1976 to William C. Blanchard and James E. Kreiner, Jr. and assigned to The Bendix Corporation. FIG. 2 shows a parabolic reflector having four dipole antennas mounted at the focal point for receiving radio signals reflected by the parabolic reflector. FIG. 3 shows a more detailed view of the dipole antennas which further includes a fifth dipole antenna mounted at the boresight of the antenna. The amplitude signals developed for the present error system are derived from the signals arriving at the antenna as the antenna is switched or scanned to provide a sum and difference mono-pulse response at four positions namely, up, right, down, left, to sequentially lobe about the target.
While the tracking of the radiosonde is based on the power of the radio signals received at the antenna, the radio signal is frequency modulated to contain the information or meteorological data which may affect the amplitude of the signal detected in the receiver. A system pointing accuracy of approximately 0.05.degree. corresponds to a 0.1 db antenna signal amplitude variation. Since synchronous detection is used, random signal fluctuations due to noise are averaged out and do not influence the performance.
The tracking system, however, is sensitive to periodic amplitude modulation when it occurs near the antenna sampling rate, typically in the range of 700 Hz to 1700 Hz or at multiples or sub-multiples thereof. The frequency modulated data results in carrier amplitude modulation which may be an order of magnitude greater than the normal tracking error signal, due to the passband of the radio frequency amplifiers in the receiver. This may result in antenna pointing errors, antenna nodding and occasional breaking of tracking lock with the radiosonde target.
Other prior art relating to frequency generation is found in U.S. Pat. No. 3,982,199 which issued on Sept. 21, 1976 to Norman Green, the inventor herein, and assigned to The Bendix Corporation. In U.S. Pat. No. 3,982,199 a digital frequency synthesizer is described incorporating an oscillator and variable frequency divider. The reference frequency from the oscillator is counted down by a variable digital frequency divider whose divisor N is selected by some means such as by switches.